Electrochemical devices such as primary batteries, secondary batteries, and electrochemical capacitors are installed in electrical devices. Such electrochemical devices are configured from components such as positive electrodes, negative electrodes, electrolytes, and separators.
The negative electrode, an essential constituting element of an electrochemical device, is preferably formed from substances that easily release electrons and provide cations. Metallic lithium is one example of the representative negative electrode materials currently in use in practical devices.
Meanwhile, in response to the recent demand for smaller and more functional electrical devices, there is a need to increase device capacity. One way to increase device capacity is to provide a negative electrode by using materials that release more electrons than does the metallic lithium, specifically substances that provide ions of higher valency than the lithium ions.
For example, JP 2007-188694A (Reference 1) proposes an electrochemical device in which metallic magnesium is used for the negative electrode. Generally, a passive coating that does not pass magnesium ions is formed on the negative electrode surface of the negative electrode active material metallic magnesium. In order to prevent formation of the passive coating, an electrolytic solution that contains alkyl aluminum is used in the electrochemical device. Specifically, the constituting elements of the electrochemical device include: a positive electrode that contains a cobalt(II) chloride active material; a metallic magnesium foil negative electrode; a polyethylene glycol separator; and an electrolytic solution that contains an ether solvent (tetrahydrofuran (THF), etc.), an electrolyte (Mg(ClO4)2, etc.) containing one or more magnesium salts, and alkyl aluminum (trimethylaluminum, etc.).
Further, a device is proposed in which a substance that does not easily form a passive coating on the negative electrode active material surface is used for the negative electrode. For example, JP 2003-100347A (Reference 2) discloses a nonaqueous electrolytic solution battery in which a magnesium alloy is used for the negative electrode. The constituting elements of the nonaqueous electrolytic solution battery include: a Fe2(SO4)3 positive electrode; a negative electrode containing one or more calcium elements and magnesium elements; a separator using materials such as polyethylene and polypropylene; and an electrolytic solution that contains a carbonate solvent (γ-butyrolactone (GBL), etc.), an electrolyte (Mg(ClO4)2, etc.) containing one or more calcium salts and magnesium salts, and other salts (additives), specifically alkylsulfone (dipropyl sulfone, etc.).
The devices using the magnesium-containing negative electrodes have a number of advantages. For example, the production cost of metallic magnesium is much lower than the cost of producing metallic lithium. Accordingly, the devices using magnesium-containing negative electrodes are less expensive than using lithium.
However, the alkyl aluminum contained in the electrolytic solution of the electrochemical device disclosed in Reference 1 lacks stability.
Reference 2 fundamentally fails to sufficiently suppress the passive coating, and is not sufficient for practical applications.